Semiconductor optical amplifier and optical module using the same

ABSTRACT

The present invention provides a polarization dependency-free, gain-saturated high function semiconductor optical amplifier and optical module at industrially low cost. The gist of the present invention is to structurally separate the optical signal propagating waveguide from another optical waveguide which serves as a lasing optical cavity for optical amplification in such a manner that the two optical waveguides are formed in the same plane but not parallel to each other.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.10/851,910 filed May 20, 2004 entitled Semiconductor Optical Amplifierand Optical Module Using the Same.

The present application claims priority from Japanese applicationJP2003-203605 filed on Jul. 30, 2003, the content of which is herebyincorporated by reference into this application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to optical communications systems. Moreparticularly, the invention relates to optical amplifiers and to modulesusing them.

2. Related Arts

Optical communications systems are rapidly becoming a widespread andimportant technology in telecommunications and networking. Opticalcommunications systems transmit information optically at veryhigh-speeds over fiber optics. The key optical components of an opticalcommunications system include optical amplifiers, in particular,semiconductor optical amplifiers. In optical communications systems,optical amplifiers are used to, for example, attenuate optical signalstherein.

There have been known semiconductor optical amplifiers. Onerepresentative type of semiconductor optical amplifier comprises anoptical cavity which resembles that of a semiconductor laser and isoperated below the lasing threshold. Another representativesemiconductor optical amplifier is a tunable-gain semiconductor opticalamplifier which controls the gain in the active region. In the formerexample, carriers are pumped by injecting current into the opticalcavity. As the optical signal passes through this region, it isamplified based on the emission stimulated by pumped carriers. Oneexample of the latter type comprises an optical cavity which lases inthe substrate's vertical direction perpendicular to the optical axis ofthe optical signal. The gain in this active region is controlled.Another gain-tunable semiconductor optical amplifier is also known whichis a gain-fixed semiconductor amplifier connected in series with avariable attenuator. These examples are disclosed in such documents asU.S. Pat. No. 6,347,104 entitled “Optical signal power monitor andregulator” (Patent Document 1) and U.S. Pat. No. 6,445,495 entitled“Tunable-gain semiconductor optical amplifier” (Patent Document 2).

[Patent Document 1]

U.S. Pat. No. 6,347,104 (lines 14 to 43 column 16, FIGS. 3A and 3B)

[Patent Document 2]

U.S. Pat. No. 6,445,495 (lines 24 to 46 column 2, FIG. 8A)

One problem with conventional optical amplifiers is that the gainchanges depending on the intensity of the incident optical signal and isnot saturate. Although gain-tunable semiconductor optical amplifiershave been proposed to cope with this problem, these examples have yet tosolve such problems as spectrum broadening due to spontaneous emissionand rising of the noise level caused by the broadening spectrum. In thecase of a semiconductor optical amplifier connected in series with avariable attenuator, it is involved with yet another problem ofincreased elements.

SUMMARY OF THE INVENTION

The above-mentioned problems are overcome by the present invention asdescribed briefly below.

In a semiconductor optical amplifier of the present invention, a firstoptical waveguide to propagate the incident optical signal and anoptical amplification section to amplify the optical signal areprovided. The optical amplification section uses only optical pumping topump carriers for stimulated emission. Stimulated emission in theoptical amplification section is introduced into the first opticalwaveguide in order to amplify the optical signal which propagatetherein. Typically, this optical pumping is done with an opticalwaveguide/cavity structure formed not parallel to the first opticalwaveguide. More specifically, laser light is obtained by the lasingoptical waveguide/cavity structure formed in the same plane but notparallel to the optical waveguide that propagates the incident opticalsignal. By the laser light going across a part or the whole of theoptical waveguide which propagates the optical signal, carriers areoptically pumped in the optical waveguide which propagates the opticalsignal. The pumped carriers stimulate emission and therefore amplify theoptical signal. The optical amplification section and the introductionof light into the first optical waveguide may be implemented in variousstyles as described later.

In a semiconductor optical amplifier of another embodiment of thepresent invention, one or more desired optical or optoelectronicparts/elements are integrated at the input end and/or output end of thefirst optical waveguide which propagates the optical signal. Theintegrated elements add new functions to the semiconductor opticalamplifier.

Yet another embodiment of the present invention is an optical moduleusing a semiconductor optical amplifier of the present invention. Thepresent invention can provide an optical module whose change of gaindepending on the intensity of the incident optical signal issubstantially eliminated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are top views of a semiconductor optical amplifieraccording to a first embodiment of the present invention;

FIG. 2 is a cross-sectional view of the semiconductor optical amplifieraccording to the first embodiment taken along line 2—2 indicated in FIG.1A;

FIG. 3 is a cross-sectional view of the semiconductor optical amplifieraccording to the first embodiment, taken along line 3—3 indicated inFIG. 1A

FIG. 4 is a cross-sectional view of the semiconductor optical amplifierof the first embodiment, taken along line 4—4 indicated in FIG. 1A;

FIG. 5 shows an example of gain characteristics of the semiconductoroptical amplifier of the first embodiment of the present invention;

FIGS. 6 is a top view of a semiconductor optical amplifier according toa second embodiment of the present invention;

FIG. 7 is a cross-sectional view of the semiconductor optical amplifieraccording to the second embodiment, taken along line 7—7 indicated inFIG. 6;

FIG. 8 is a cross-sectional view of the semiconductor optical amplifieraccording to the second embodiment, cut along line 8—8 indicated in FIG.6;

FIG. 9 is a top view of a semiconductor optical amplifier according to athird embodiment of the present invention;

FIG. 10 is a cross-sectional view of the semiconductor optical amplifieraccording to the third embodiment, taken along line 10—10 indicated inFIG. 9;

FIG. 11 is a cross-sectional view of the semiconductor optical amplifieraccording to the third embodiment, taken along line 11—11 indicated inFIG. 9;

FIG. 12 is a cross-sectional view of another semiconductor opticalamplifier according to the third embodiment, taken along line 10—10indicated in FIG. 9;

FIG. 13 is a cross-sectional view of another semiconductor opticalamplifier according to the third embodiment, taken along line 11—11indicated in FIG. 9;

FIG. 14 is a top view of a semiconductor optical amplifier according toa fourth embodiment of the present invention;

FIG. 15 is a cross-sectional view of the semiconductor optical amplifieraccording to the fourth embodiment, taken along line 15—15 indicated inFIG. 14;

FIG. 16 shows a cross-sectional view of the semiconductor opticalamplifier according to the fourth embodiment, taken along line 16—16indicated in FIG. 14;

FIG. 17 is a top view of a semiconductor optical amplifier in which avariable attenuator is integrated at the input end of the semiconductoroptical amplification section, according to a fifth embodiment of thepresent invention;

FIG. 18 is a cross-sectional view of the semiconductor optical amplifieraccording to the fifth embodiment in which a variable attenuator isintegrated at the input end of the semiconductor optical amplificationsection, taken along line 18—18 indicated in FIG. 17;

FIG. 19 is a top view of another semiconductor optical amplifier inwhich a variable attenuator is integrated at the output end of thesemiconductor optical amplification section, according to the fifthembodiment of the present invention;

FIG. 20 shows a cross-sectional view of the semiconductor opticalamplifier according to the fifth embodiment of the present invention inwhich a variable attenuator is integrated at the output end of thesemiconductor optical amplification section, taken along line 20—20indicated in FIG. 19;

FIG. 21 is a top view of a semiconductor optical amplifier in which aphoto acceptance element is integrated, according to a sixth embodimentof the present invention;

FIG. 22 shows a cross-sectional view of the semiconductor opticalamplifier according to the sixth embodiment in which a photo acceptanceelement is integrated, taken along 22—22 indicated in FIG. 21;

FIG. 23 schematically shows the configuration of a module using thesemiconductor optical amplifier according to the sixth embodiment of thepresent invention in which a photo acceptance is integrated;

FIG. 24 schematically shows the configuration of another module usingthe semiconductor optical amplifier according to the sixth embodiment ofthe present invention in which a photo acceptance is integrated;

FIG. 25 shows an example of characteristic, as a reception module, ofthe semiconductor optical amplifier according to the sixth embodiment inwhich a photo acceptance is integrated;

FIG. 26 shows the configuration of a semiconductor optical amplifiermodule according to a seventh embodiment of the present invention;

FIG. 27 shows another configuration of a semiconductor optical amplifiermodule according to the seventh embodiment of the present invention; and

FIG. 28 shows another configuration of a semiconductor optical amplifiermodule according to the seventh embodiment of the present invention.

PREFERRED EMBODIMENT OF THE INVENTION

Before proceeding to specific embodiments, the following provides adetailed description of general matters concerning the presentinvention. FIG. 1A is a top view of a representative embodiment of thepresent invention. Note that although the present invention isconcretely described by using the embodiment of FIG. 1A, this does notmean the principle of the present invention is applied only to thisembodiment. There are provided a first optical guide 101 to guide anincident signal 170, and optical waveguide region for lasting 102 thatare formed in the same plane as but not parallel to the first opticalguide 101. Each layer of them can be formed by ordinary semiconductorprocess. Note that in the interest of process operation, thesemiconductor optical cavity is sometimes formed such that its incidenceand emission facets are not perpendicular to, that is, lie at an angleto the optical axis of the first optical waveguide. In this case, due tothe dependence of etching on the crystal structure, grooves to formreflectors for lasing oscillation become not perpendicular to thefacets. FIG. 1B is a top view of this configuration. Each part issimilar to that in FIG. 1A. In FIG. 1B, the angle between the extensionof the optical axis of the first optical waveguide and the extension ofa side, closer to the first optical waveguide, of each groove to form areflector for lasing oscillation is schematically depicted asintersection angle θ. Practically, considering the relationship betweenair, the refractive index of the InP based compound semiconductor andthe Brewster angle, this intersection angle θ is set in the range ofabout 4 to 7 degrees.

Typically, the optical waveguide at least includes a core layer and twocladding layers which sandwiches the core layer. Further, the core layerand other layers may employ a multi quantum well structure. For example,a multi quantum well structure is formed by repeatedly stackingInGaAs/InGaAsP.

This optical cavity structure can stage lasing oscillation. By lettinglaser light go across a part or the whole of the optical signalpropagation waveguide, the optical cavity structure optically pumpscarries in the optical signal propagation waveguide in order to causestimulated emission there. The optical signal is amplified by thisstimulated emission.

In this case, pumped carriers are consumed for amplification butimmediately replenished by the incident laser light from the directiontransverse to the propagating direction of the optical signal. Thepumped carrier population, that is, the population inversion is keptconstant not depending on the power level of the input optical signal.Thus, it is possible to saturate the gain as a semiconductor opticalamplifier.

In addition, since the optical waveguide region for lasing 102 and theoptical signal propagation optical waveguide regions 101, each made of asemiconductor multi-layered structure, are separated structurally fromeach other, the section of the optical signal propagation waveguide cutperpendicular to the propagating direction (optical axis) of light canbe made substantially square. Therefore, any section perpendicular tothe propagating direction of light has a circular area where the gain issubstantially uniform for any direction in the plane. Thus, this cansolve the problem that conventional semiconductor optical amplifiershave polarization dependency, that is, take gains differing independence on polarization.

The structural separation of the optical signal propagating waveguidefrom the lasing waveguide according to the present invention is alsoeffective in minimizing the broadening of the spectrum and theaccompanied rising noise level. If a surface emission laser is used,current is injected into the optical signal waveguide, too. In thiscase, pumping is partly made by the current, which causes spontaneousemission and therefore broadens the spectrum and raises the noise level.

In addition, it is necessary to lower the resistance of the current pathby doping p-type or n-type impurities into the cladding layers up to acertain density level since the current path goes through the signalpropagation waveguide. In the case of the structure according to thepresent invention, since the optical signal is separated from the lasingwaveguide, it is not necessary to dope the cladding layers of theoptical signal propagation waveguide. Loss due to impurities cantherefore be prevented.

Note that in the embodiment of the present invention, the wavelength ofthe optical signal is set equal to or shorter than the compositionwavelength of the medium of the relevant optical waveguide; the lasingwavelength for optical amplification is set equal to or shorter than thecomposition wavelength of the medium of the relevant optical waveguide;and, the lasing wavelength is set equal to or shorter than the opticalsignal wavelength. That is, these relations can be represented by:[Laser Light Wavelength]≦[Optical Signal Wavelength]≦[CompositionWavelength of Optical Waveguide Medium]. Preferably, the optical signalwavelength is equal to the composition wavelength of the opticalwaveguide medium.

The optical feedback parts to constitute an optical cavity using thesecond optical waveguide in accordance with the present invention cansatisfactorily be fabricated by such methods as employed for ordinarysemiconductor lasing cavities. One representative example method is toform reflective surfaces either by dry etching or cleaving both thefacets. After that, either a dielectric multi-layered film or asemiconductor multi-layered mirror film is typically formed on thesereflective surfaces. The present invention also allows this embodimentto be modified such that the optical feedback means is implemented byforming a grating either in each semiconductor layer of the secondoptical waveguide or in a region where the laser light is sensitive.

In addition to using the second optical waveguide, constituting anoptical cavity, provided on both the sides of the first opticalwaveguide, the present invention also allows the perpendicularmulti-layered structure region of the semiconductor optical amplifier tobe used partly in constituting an optical cavity. That is, a 45-degreereflector mirror is formed along each facet of the second opticalwaveguide so that light can be reflected toward the substrate of thesemiconductor optical amplifier by each 45-degree reflector mirror. Thenan optical cavity is constituted by forming a reflecting part in a lightpath on the substrate side for each reflector mirror. In many cases, thereflecting parts are formed on the top or bottom side of the substratesince they are easy to form there. Together with them, an appropriateregion of the multi-layered structure of the second optical waveguide orthe semiconductor optical amplifier is used to constitute an opticalcavity. Note that the 45-degree reflector mirror means a reflectormirror which makes an angle of 45 degrees with the propagating directionof light in the second optical waveguide. For example, this 45-degreereflector mirror can be formed through crystal orientation-dependent wetetching of the semiconductor multi-layered structure.

Further, needless to say, the present invention allows the semiconductoroptical amplifier to incorporate one or more desired optical oroptoelectronic parts/elements at the input end and/or output end of thefirst optical waveguide or optical signal propagation waveguide.

Specific examples are as follows: A first example is shown in FIG. 17where an electro absorption type VOA (Variable Optical Attenuator) isfabricated in the front of the optical signal propagation waveguide inorder to control the input optical power level. A second example isshown in FIG. 20 where an electro absorption type VOA is fabricated inthe rear of the optical signal propagation waveguide in order to controlthe output optical power level. A third example is shown in FIG. 21where a PIN photodiode is fabricated in the rear of the optical signalpropagation in order to constitute an optical preamplifier.

Major modes of the present invention are summarized below.

A first mode is a semiconductor optical amplifier comprising a firstoptical waveguide which propagates an input optical signal; and anoptical amplification section, which amplifies the optical signal bycausing stimulated emission with radiation incident on the first opticalguide or with a cavity structure formed in the direction whichintersects the optical propagating direction of the first opticalwaveguide.

A second mode is that any gain medium to amplify light is not providedaround the optical input surface of the first optical waveguide whichpropagates the input optical signal.

A third mode is that a part or the whole of the gain medium of the firstoptical waveguide which propagates the input optical signal is used alsoas a part or the whole of the gain medium of the optical amplificationsection which amplifies the optical signal.

A fourth embodiment is that a part or the whole of the gain medium ofthe first optical waveguide which propagates the input optical signal isused also as a part of the gain medium of the optical amplificationsection which amplifies the optical signal; or the gain medium used inthe first optical waveguide is different in composition from the gainmedium used in a portion which generates radiation incident on the firstoptical waveguide.

A fifth mode is that photonic crystal is provided along each side of thefirst optical waveguide which propagates the input optical signal; andwith a cavity structure formed in the direction which intersects theoptical propagating direction of the first optical waveguide, theoptical signal is amplified by causing stimulated emission.

A sixth mode is that, without injecting current into the first opticalwaveguide which propagates the input optical signal, the optical signalis amplified by causing stimulated emission by optically pumpingcarriers in the first optical waveguide with radiation incident on thefirst optical waveguide from the direction which is included in a planeparallel to the first optical waveguide and intersects the opticalpropagating direction of the first optical waveguide.

Major modes of the present are descried so far. Needless to say, thesemodes may be combined as required by the device to be implementedaccording to the present invention.

Embodiment 1

The following describes a semiconductor optical amplifier having onelasing cavity structure according to a first embodiment with referenceto FIG. 1 through FIG. 4. As described earlier, FIG. 1A is a top view.FIG. 2 shows a section taken along line 2—2 indicated in FIG. 1. FIG. 3shows a section taken along line 3—3 indicated in FIG. 1. FIG. 4 is asection taken along line 4—4 indicated in FIG. 1.

Referring to FIG. 1A, the following describes the basic operation ofthis embodiment. From the left side, an optical signal 170 is input toan optical waveguide 101 shown in the center of the figure. The opticalsignal 170 is amplified as it passes through an optical waveguide regionfor lasing 102, and is output as output light 171 from the right side ofthe optical waveguide 101. In FIG. 1, reference numeral 101 denotes anoptical waveguide for the optical signal, 102 is an optical waveguideregion for lasing, 103 is a bottom electrode pad, 104 is anantireflection coating formed on each end surface of the optical signalwaveguide, and 105 is a coating for the lasing cavity. In thisembodiment, the crystal facets to form the lasing cavity are obtained bydry etching although they may also be obtained by cleaving.

Shown in FIG. 3 is a section taken along 3—3 indicated in FIG. 1. Themulti-layered structure of the optical signal waveguide 101 is formed onan InP substrate (n-type, 2×10¹⁸ cm⁻³, 100 μm) 111 by stacking an InPbuffer layer (n-type, 1×10¹⁸ cm⁻³, 0.15 μm) 112, an InGaAsP claddinglayer (none-dope, thickness 0.3 μm, λg=1.15 μm) 113, an InGaAsPwaveguide layer (none-dope, thickness 0.8 μm, λg=1.55 μm) 114, anInGaAsP cladding layer (none-dope, thickness 0.3 μm, λg=1.15 μm) 115, anInP cap layer (none-dope, thickness 1.0 μm) 116 and an insulating film(SiN, thickness 0.5 μm) 117. Above, and throughout the remainder of thisdescription, whenever information is provided within parentheses, itshould be interpreted as first specifying the dopant type, second theimpurity concentration of that dopant, and third the thickness of thelayer to which reference is made.

Shown in FIG. 2 is a section along 2—2 indicated in FIG. 1. Referring toFIG. 2, the multi-layered structure of the lasing cavity part isdescribed. The lasing cavity part is formed so as to sandwich theoptical waveguide part 101 from both sides. A lasing cavity isconstituted by the optical waveguide part 101 and the multi-layeredstructure described below. The axial direction of the lasing cavityintersects with the propagating direction of the optical signal.

The multi-layered structure which sandwiches the optical waveguide part101 from both sides is fabricated by stacking an InP buffer layer(n-type, 1×10¹⁸ cm⁻³, thickness 0.15 μm) 112, an InGaAsP cladding layer(n-type, 5×10¹⁸ cm⁻³, thickness 0.2 μm, ëg=1.05 μm) 122, an InGaAsP SCHlayer (n-type, 1×10¹⁷ cm⁻³, thickness 0.2 μm, ëg=1.15 μm) 123, anInGaAsP MQW (Multi-quantum Well) active layer (none-dope, well layerthickness 10 nm/ëg=1.55 μm, barrier layer thickness 10 nm/ëg=1.3 μm, 10periods) 124, an InGaAsP SCH layer (p-type, 1×10¹⁷ cm⁻³, thickness 0.2μm, ëg=1.15 μm) 125, an InP cladding layer (p-type, 5×10¹⁷ cm⁻³,thickness 1.5 μm) 128, an InGaAs contact layer (p-type, 2×10¹⁹ cm⁻³,thickness 0.1 μm) 129, an insulation film (SiN, thickness 0.5 μm) 117and a p electrode (Ti/Pt/Au) 118 on the InP substrate 111. On the bottomof the substrate 111, a n electrode (Ni/AuGe/Au) 119 is formed.

Note that a semiconductor layer (for example a barrier layer) may beinserted between the semiconductor multi-layered region constituting thefirst optical waveguide and the semiconductor multi-layered regionconstituting the second optical waveguide in order to block thediffusion of impurities. Not limited to this embodiment, thissemiconductor layer may be added to the other embodiments of the presentinvention.

FIG. 4 shows a longitudinal section of the optical signal waveguidetaken along the direction of propagation. The same multi-layeredstructure as the optical waveguide in FIG. 2 and FIG. 3 is shown.Therefore, this figure is not described in detail.

The following describes an example of a method for fabricate theaforementioned semiconductor multi-layered structures. Firstly, thefirst semiconductor multi-layered structure constituting the firstoptical waveguide is formed on the substrate 111. The firstsemiconductor multi-layered structure may be formed wider and processedto a desired width. It is also possible to stack the respective layershaving the desired width. Then, the second semiconductor multi-layeredstructure constituting the aforementioned optical amplifier part isformed in parallel with the optical axis of the first semiconductormulti-layered structure and in contact with the longitudinal sidesthereof. After the first and second semiconductor multi-layeredstructures are shaped as desired, a semi-insulation semiconductor layer130, such as a semi-insulation InP buried layer, is formed so as tosurround them. Then, after groove parts are formed along the boundariesof the semi-insulation semiconductor layers 130 surrounding the secondsemiconductor multi-layered structure, a coating film 105 is depositedin the groove parts to form a lasing cavity. These groove parts can beformed either by dry etching or cleaving as mentioned earlier. Note thatthis coating film 105 can be deposited in other regions unless improperin terms of manufacture although the coating film 105 is notparticularly required besides both sides of the lasing cavity. Further,an antireflection coating 104 is formed over each facet of the firstoptical waveguide which propagates the optical signal. Then, a p-typeelectrode 118 and a n-type electrode 119 are formed to complete anoptical amplifier.

The gain characteristic of this semiconductor amplifier according to thepresent invention was measured. The semiconductor amplifier was set on asub-mount and aligned with lenses and fibers. FIG. 5 shows some of theresult. The horizontal axis represents the optical output power whereasthe vertical axis represents the gain. As shown in FIG. 5, it isverified that the gain, or the ratio of the output signal to the inputsignal, is almost constant or saturated at about 16 dB not depending onthe power level of the input optical signal. It is also verified thatthe gain is free from polarization dependency thanks to thesubstantially square cross section of the optical waveguide whichpropagates the optical signal.

In addition, advantages are brought about by the structural separationof the lasing optical waveguide from the optical signal propagationwaveguide. To be more specific, since no current is injected into theoptical signal propagation waveguide, spectrum broadening due tospontaneous emission can be minimized, which results in a lower level ofnoise. In addition, since the cladding layers in the optical signalpropagation waveguide are not doped in this optical amplifier, loss dueto impurities in the optical waveguide can be prevented.

In addition, since an optical waveguide structure, instead of a surfaceemission structure, is employed as the cavity structure to generatepumping laser light, it is possible to raise the intensity of thepumping light.

Embodiment 2

With reference to FIG. 6 through FIG. 8, the following describes anoptical amplifier having a plurality of lasing cavity structuresaccording to a second embodiment. FIG. 6 shows a top view of thisoptical amplifier. FIG. 7 shows a section taken along line 7—7 indicatedin FIG. 6. FIG. 8 shows a section taken along line 8—8 indicated in FIG.6.

From the left side, an optical signal 170 enters an optical waveguide101 shown in the center of the figure. The optical signal is amplifiedas it passes through a plurality of separate lasing cavities, andemitted as output light 171 from the right side of the opticalwaveguide.

Each of the plural optical waveguide region for lasing cavities is shownas a shaded part 102 in FIG. 6. Several methods can be used to form theplurality of separate lasing cavities. One of the methods is describedbelow as an example. That is, after a multi-layered structure is formed,the structure is partitioned into separate ones by forming grooves. Thesame layer as for the optical signal propagation waveguide part isburied later in these grooves by epitaxial growth. These separatingregions may also be left as grooves between ridges. In this case, theseparating grooves are only passivated internally with a protection filmafter they are formed.

In the figures, 101 denotes the optical waveguide for the opticalsignal, 103 is an electrode pad, 104 is an antireflection coating formedon each end surface of the optical signal propagation waveguide and 105is a coating for the lasing cavities.

In this embodiment, the crystal facets to constitute the lasing cavitiesare obtained by dry etching. They may also be obtained by cleaving, acommon method. As for the multi-layered structure constituting theoptical signal propagation waveguide, the multi-layered structureconstituting the lasing cavities and the electrodes, their detaileddescription is omitted here since they are identical to those in thefirst embodiment.

As mentioned earlier, FIG. 7 shows a section which includes the signalwaveguide part 7 and the laser part whereas FIG. 8 shows a section whichincludes the signal waveguide part but not the laser part. Separationinto plural lasing cavities is made in order to narrow the width of eachlasing cavity. This stabilizes the transverse mode and thereby raisesthe laser part's linearity of the relation between the bias current andthe pumping optical power.

The gain characteristic of this semiconductor amplifier according to thepresent invention was measured. An optical signal is input to thesemiconductor amplifier which was set on a sub-mount and aligned withlenses and fibers. Similar to the characteristic shown in FIG. 5, it isverified that the gain is saturated. It is also verified that the gainis free from polarization dependency thanks to the substantially squarecross section of the optical waveguide which propagates the opticalsignal.

Further, advantages are brought about by the structural separation ofthe lasing optical waveguide from the optical signal propagationwaveguide. That is, since no current is injected into the optical signalpropagation waveguide, spectrum broadening due to spontaneous emissioncan be suppressed, which results in a lower level of noise. In addition,since the cladding layers in the optical signal propagation waveguideare not doped in this optical amplifier, loss due to impurities in theoptical waveguide can be prevented.

In addition, since an optical waveguide structure, instead of a surfaceemission structure, is employed as the cavity structure to generatepumping laser light, it is possible to raise the intensity of thepumping light.

Embodiment 3

In a third embodiment, 45-degree reflecting mirrors are used for anoptical amplifier. That is, a lasing cavity to generate pumping lightused to amplify the optical signal is constituted by multi-layereddielectric films and an optical waveguide formed on the bottom side ofthe substrate and two 45-degree reflecting mirrors on the top side ofthe substrate. This embodiment is described with reference to FIG. 9through FIG. 11. FIG. 9 is a top view of this optical amplifier. FIG. 10shows a section taken along line 10—10 indicated in FIG. 9, that is,this figure shows a section which includes the lasing cavity part. FIG.11 shows a section taken along line 11—11. The longitudinal section ofthe optical signal propagation waveguide part, taken along line 4—4, isthe same as shown in FIG. 4. The multi-layered structures whichrespectively constitute the optical signal propagation waveguide partand the lasing cavity part are identical to those in the firstembodiment.

Referring to FIG. 9, an optical signal 170 enters an optical waveguide101 from the left entrance. The optical signal 170 is amplified as itpasses through the lasing cavity, and emitted as output light 171 fromthe right side of the optical waveguide. In the figure, 101 denotes theoptical waveguide for the optical signal, 103 is an electrode pad formedon the bottom side, 104 is an antireflection coating formed on eachfacet of the optical signal propagation waveguide and 131 is a 45-degreemirror part. Each 45-degree mirror part 131 is formed by wet-etching thesemiconductor multi-layered structure at 45 degrees to the substrate anddepositing a high reflectance dielectric film 132 on the obtained45-degree surface. This is because numerals 131/132 are used in thefigure to denote the relevant regions. High reflection dielectric films133 are formed on the bottom side of the substrate by mirror finishetching treatment. The thus formed two 45-degree mirror parts 131, thesemiconductor multi-layered structure and the reflection parts 133 ofthe substrate bottom constitute an optical cavity.

FIG. 12 shows a section of a modification of the third embodiment. Inthis modification, the reflector parts 133 are formed as a semiconductormulti-layered reflector film 134 on the substrate 111. The n-typesemiconductor multi-layered reflector film is formed before the n-typecladding layer 113 is deposited above. This eliminates the necessity offorming reflector parts on the bottom side of the substrate. EitherInGaAsP/InP or GaAs/InAs may be used to form the semiconductormulti-layered reflector film.

The largest structural advantage of this embodiment is that the lasingcavity can be formed without using such advanced techniques asdeposition of a multi-layered dielectric reflector film on aperpendicular surface formed by dry etching.

The gain characteristic of the third semiconductor amplifier embodimentaccording to the present invention was measured. An optical signal isinput to the semiconductor amplifier which was set on a sub-mount andaligned with lenses and fibers. Similar to the characteristic shown inFIG. 5, it is verified that the gain is saturated. It is also verifiedthat the gain is free from polarization dependency thanks to thesubstantially square cross section of the optical waveguide whichpropagates the optical signal.

Further, advantages are brought about by the structural separation ofthe lasing optical waveguide from the optical signal propagationwaveguide. Namely, since no current is injected into the optical signalpropagation waveguide, spectrum broadening due to spontaneous emissioncan be suppressed, which results in a lower level of noise. In addition,since the cladding layers in the optical signal propagation waveguideare not doped in this optical amplifier, loss due to impurities in theoptical waveguide can be prevented.

In addition, since an optical waveguide structure, instead of a surfaceemission structure, is employed as the cavity structure to generatepumping laser light, it is possible to raise the intensity of thepumping light.

Embodiment 4

In a fourth embodiment, a grating is formed in an optical waveguide partwhich constitutes a lasing cavity used to amplify an optical signal.This embodiment is described with reference to FIG. 14 through and FIG.16. FIG. 14 is a top view of this optical amplifier. FIG. 15 shows asection of the optical signal propagation waveguide part taken alongline 15—15 indicated in FIG. 14. FIG. 16 shows a section of the lasingcavity part taken along line 16—16 indicated in FIG. 14. Themulti-layered structure of the optical signal propagation waveguide partis basically identical to that in the first embodiment.

From the left side, an optical signal 170 is input to an opticalwaveguide 101 shown in the center of the figure. The optical signal 170is amplified as it passes through an optical waveguide region for lasing102, and is output as output light 171 from the right side of theoptical waveguide 101. In the figure, reference numeral 101 denotes anoptical waveguide for the optical signal, 102 is an optical waveguideregion for lasing, 103 is a bottom electrode pad, 104 is anantireflection coating formed on each facet of the optical signalwaveguide, and 105 is a coating for the lasing cavity.

In the lasing cavity of this embodiment, a grating 127 is formed betweenan SCH (Separate Confining Heterostructure) layer 125 and a claddinglayer 128 in order to select a longitudinal mode. In addition, thecrystal facets to constitute the lasing cavity are obtained by dryetching although they may also be obtained by cleaving. Although thegrating 127 enables lasing, these reflecting facets are used effectivelyto confine laser light in the chip and thereby raise the efficiency.

FIG. 15 shows a section of the lasing cavity part. As shown, the laserpart is a multi-layered structure formed by stacking on an InP substrate111 an InP buffer layer (n-type, 1×10¹⁸ cm⁻³, thickness 0.15 μm) 112, anInGaAsP cladding layer (n-type, 5×10¹⁷ cm⁻³, thickness 0.2 μm, ëg=1.05μm) 122, an InGaAsP SCH layer (n-type, 1×10¹⁷ cm⁻³, thickness 0.1 μm,ëg=1.15 μm) 123, an InGaAsP MQW active layer (none-dope, well layerthickness 10 nm/ëg=1.55 μm, barrier layer thickness 10 nm/ëg=1.3 μm, 10periods) 124, an InGaAsP SCH layer (p-type, 1×10¹⁷ cm⁻³, thickness 0.1μm) 125, an InP spacer layer (p-type, 5×10¹⁷ cm⁻³, thickness 0.2 μm)126, an InGaAsP grating layer (p-type, 5×10¹⁸ cm⁻³, thickness 0.05 μm)127, an InP cladding layer (p-type, 1×10¹⁷ cm⁻³, thickness 1.5 μm) 128,an InGaAs contact layer (p-type, 2×10¹⁹ cm⁻³, thickness 0.1 μm) 129, aninsulation film (SiN, thickness 0.5 μm) 117 and a p electrode (Ti/Pt/Au)118 on the InP substrate 111. Further, an n-electrode (Ni/AuGe/Au) 119is formed on the bottom of the substrate 111. The grating is formed byelectron beam lithography.

The gain characteristic of this semiconductor amplifier according to thepresent invention was measured. An optical signal is input to thesemiconductor amplifier which was set on a sub-mount and aligned withlenses and fibers. Similar to the characteristic shown in FIG. 5, it isverified that the gain is saturated. It is also verified that the gainis free from polarization dependency thanks to the substantially squarecross section of the optical waveguide which propagates the opticalsignal.

Further, advantages are brought about by the structural separation ofthe lasing optical waveguide from the optical signal propagationwaveguide. That is, since no current is injected into the optical signalpropagation waveguide, spectrum broadening due to spontaneous emissioncan be suppressed, which results in a lower level of noise. In addition,since the cladding layers in the optical signal propagation waveguideare not doped in this optical amplifier, loss due to impurities in theoptical waveguide can be prevented.

In addition, since an optical waveguide structure, instead of a surfaceemission structure, is employed as the cavity structure to generatepumping laser light, it is possible to raise the intensity of thepumping light.

Embodiment 5

With reference to FIGS. 17 and 18, the following describes a fifthembodiment having a variable optical attenuator integrated to theentrance of the optical signal propagation waveguide. FIG. 17 is a topview of this optical amplifier while FIG. 18 shows a section taken alongline 18—18 indicated in FIG. 17 parallel to the propagating direction oflight. From the left side, an optical signal 170 enters a variableoptical attenuator 106 through which the optical power level isadjusted. Then the optical signal enters the optical waveguide of theoptical amplifier 107 and is amplified as it passes through the opticalwaveguide region for lasing 102. The amplified optical signal is emittedas output light 171 from the right side of the optical waveguide. Whenplural optical signals having different levels of optical power aretreated in parallel, it is possible to make the output signals uniformin power by adjusting the input power levels to the same level.

FIG. 18 shows a section of the variable optical attenuator. Itsmulti-layered structure is formed by stacking an InP buffer layer(n-type, 1×10.sup.18 cm⁻³, thickness 0.15 μm) 112, an InGaAsP bufferlayer (n-type, 5×10¹⁷ cm⁻³, thickness 0.2 μm, ëg=1.05 μm) 152, anInGaAsP SCH layer (n-type, 1×10¹⁷ cm⁻³, thickness 0.2 μm, ëg=1.15 μm)153, an InGaAsP MQW active layer (none-dope, well layer thickness 8nm/ëg=1.52 μm, barrier layer thickness 12 nm/ëg=1.3 μm, 10 periods) 154,an InGaAsP SCH layer (none-dope, thickness 0.2 μm, ëg=1.10 μm) 155, anInP cladding layer (p-type, 1×10¹⁸ cm⁻³, thickness 1.5 μm) 158, anInGaAsP contact layer (p-type, 2×10¹⁹ cm⁻³, thickness 0.1 μm) 159, aninsulation film (SiN, thickness 0.5 μm) 117 and a p electrode (Ti/Pt/Au)118 on the InP substrate 111. On the bottom of the substrate 111, ann-electrode (Ni/AuGe/Au) 119 is formed.

FIGS. 19 and 20 show an embodiment having a variable attenuatorintegrated at the exit. FIG. 19 is its top view while FIG. 20 shows asection. It has the same structure as the above-mentioned embodimentexcept that a variable attenuator 106 is formed at the exit. The regionof the variable attenuator 106 is substantially the same as the variableoptical attenuator in FIG. 18. The power level of the optical signal 170amplified by the semiconductor amplifier can be adjusted to anappropriate level by the variable attenuator at the exit. Therefore,when plural optical signals having different levels of power are treatedin parallel, it is possible, for example, to make the individual powerlevels uniform at the exit. It is also possible to cut off the signalsof specific channels.

Further, if a high-speed variable attenuator, namely, an EA modulator isintegrated at the exit in the semiconductor optical amplifier, it ispossible to generate a large amplitude optical signal by amplifying CW(Continuous Wave) light.

Embodiment 6

In a sixth embodiment, a photo acceptance element is integrated at theexit of the optical waveguide which propagates the optical signal. FIG.21 is a top view thereof. FIG. 22 shows a section thereof taken alongline 22—22 of FIG. 21 parallel to the propagating direction of light.The optical signal 170 enters the optical waveguide of the opticalamplifier 107 from the left side and is amplified as it passes throughthe optical waveguide region for lasing 102. Then, the amplified opticalsignal enters the photo acceptance element 108 integrated at the rightend of the optical waveguide 107 and is converted to an electricalsignal. Thanks to amplification by the optical amplifier, even a subtleoptical signal can be amplified to exceed the minimum level sensible bythe photo acceptance element.

FIG. 22 shows the multi-layered structure of the photo acceptanceelement. On the InP substrate 111, stacked are an InP buffer layer(n-type, 1×10¹⁸ cm⁻³, 0.15 μm) 112, an InGaAsP cladding layer (n-type,none-dope, 1×10¹⁷ cm⁻³, thickness 0.5 μm, ëg=1.15 μm) 163, an InGaAsabsorption layer (none-dope, thickness 1.5 μm) 164, an InGaAsP claddinglayer (p-type, 1×10¹⁷ cm⁻³, thickness 0.2 μm, ëg=1.15 μm) 165, anInGaAsP cap layer (p-type, 1×10¹⁸ cm⁻³, thickness 0.2 μm) 166, an InGaAscontact layer (p-type, 2×10¹⁹ cm⁻³, thickness 0.1 μm) 167, an insulatingfilm (SiN, thickness 0.5 μm) 117 and a p-electrode (Ti/Pt/Au). On thebottom of the substrate 111, an n-electrode (Ni/AuGe/Au) 119 is formed.

FIG. 23 shows an example of an optical reception module configuration inwhich a semiconductor optical amplifier incorporating a photo acceptanceelement is combined with a lens 210 and an optical fiber 211. FIG. 24shows an example of an optical reception module configuration in whichthe semiconductor optical amplifier is further combined with apreamplifier 109. In FIG. 23, the optical signal 170 is introduced intothe optical fiber 211 from the entrance 121 of the optical receptionmodule. The optical signal is converged into the optical amplificationpart of the semiconductor optical amplifier 107 through the lens 210.Further, the output from the optical amplification part is input to thephoto acceptance element 108, namely such as a PIN photodiode. Then, thesignal is taken out from the photo acceptance element 108 as anelectrical signal. The optical reception module in FIG. 24 is identicalto that in FIG. 23 except that the preamplifier 109 is provided for theelectrical output from the photo acceptance element 108. Note that eachblack circle in the figures means an electrical connection or aterminal.

In such an embodiment, the semiconductor optical amplifier can amplify asubtle optical signal which cannot be received by an ordinary receptionmodule consisting merely of a photo acceptance device and apreamplifier. Therefore, it is possible to provide raised totalreception performance as a reception module. An example of the totalreception performance as a reception module is shown in FIG. 25. Thehorizontal axis represents the receiving sensitivity while the verticalaxis represents the BER. A characteristic 140 is that of a receptionmodule having a SOA (Semiconductor Optical Amplifier) and a PINphotodiode in accordance with the present invention while acharacteristic 141 is that of a reception module having a PIN photodiodeand a preamplifier with no SOA. According to these examples ofcharacteristics, 10 dB or more improvement is obtained if asemiconductor optical amplifier is included, making it possible toattain a high level of reception performance comparable to that obtainedby using an APD. Therefore, these reception modules in accordance withthe present invention shows the effect of integration particularly inhigher-than-10 Gbps applications where high level packaging technologyis required.

Embodiment 7

FIG. 26 shows an example of a module configured by combining one of thesemiconductor optical amplifiers of the first to fourth embodiments witha lens 210, a fiber 211 and a Peltier device 212. This configurationmakes it possible to manufacture a gain saturated, low coupling lossmodule. Note that identical reference numerals are used to designatethose identical to their corresponding ones in FIGS. 23 and 24.Reference numeral 122 designates the output terminal.

In the module of FIG. 27, a variable attenuator-integrated semiconductoroptical amplifier is combined with a control switch. The module in FIG.28 has an EA modulator-integrated semiconductor optical amplifiercombined with a driver IC. Including them, a variety of alterations andconfigurations are possible. Accordingly, by use of such a chip devicewhere needed electrical and/or optical elements are integrated on asubstrate, it is possible to provide a low packaging cost, low couplingloss tunable semiconductor amplifier module.

Note that although InGaAsP, InGaAs and InP are used as layered crystalsin the first through seventh embodiments, other crystal systems such asInAlGaAs and InAlAs can also be used as layered crystals. Needless tosay, it is also possible to freely determine the types (p-type orn-type) of the substrate and each layer and the densities of impuritiestherein as needed by the application.

Problems with prior art semiconductor optical amplifiers are that gainshows polarization dependency and is not saturated and integrating asurface emitting laser in an optical amplifier in order to saturate thegain results in higher cost due to long epitaxial growth time. Asdescribed so far based on various embodiments, the present inventionsolves these problems by forming a structurally separate lasing opticalwaveguide in the same plane as but not in parallel to the optical signalpropagation waveguide. The present invention makes it possible toprovide an inexpensive, gain-saturated, polarization dependency-freehigh-function semiconductor optical amplifier/module. Industrially, thepresent invention has great importance.

Major embodiments of the present invention are summarized and listed asfollows:

(1) A semiconductor optical amplifier comprising an optical waveguidewhich propagates an input optical signal and having a function toamplify the signal, wherein the optical signal is amplified bystimulating emission in the optical waveguide with carriers which arepumped fully optically.

(2) A semiconductor optical amplifier according to Paragraph (1),wherein: an optical waveguide/cavity structure is formed in the sameplane but not parallel to the optical waveguide which propagates theinput optical signal; and the optical signal is amplified by stimulatingemission in the optical waveguide with carriers pumped by laser lightwhich is generated by the lasing optical waveguide/cavity structure andgoes across a part or the whole of the optical wavelength.

(3) A semiconductor optical amplifier wherein: as stated in Paragraph(2), an optical waveguide/cavity structure is formed in the same planebut not parallel to the optical waveguide which propagates the inputoptical signal; as stated in Paragraph (2), the optical signal isamplified by stimulating emission in the optical waveguide with carrierspumped by laser light which is generated by the lasing opticalwaveguide/cavity structure and goes across a part or the whole of theoptical wavelength; and the lasing optical waveguide/cavity structure isseparated into a plurality of optical waveguide/cavity units.

(4) A semiconductor optical amplifier according to Paragraph (2) or (3)wherein the optical reflectors to constitute the lasing cavity structureused to amplify the optical signal is obtained by depositing adielectric multi-layered film on facets formed by dry etching.

(5) A semiconductor optical amplifier according to Paragraph (2) or (3)wherein the optical reflectors to constitute the lasing cavity structureused to amplify the optical signal is obtained by depositing adielectric multi-layered film on facets formed by cleaving.

(6) A semiconductor optical amplifier according to Paragraph (2) or (3)wherein the optical reflectors to constitute the lasing cavity structureused to amplify the optical signal are dielectric multi-layered filmsformed on the bottom side of the substrate and two 45-degree reflectormirrors formed on the same side of the substrate as the opticalwaveguide.

(7) A semiconductor optical amplifier according to Paragraph (2) or (3)wherein the optical reflectors to constitute the lasing cavity structureused to amplify the optical signal are semiconductor multi-layered filmsformed by epitaxial growth and two 45-degree reflector mirrors formed onthe same side of the substrate as the optical waveguide.

(8) A semiconductor optical amplifier according to any of Paragraphs (1)through (7) wherein a grating is formed in an optical waveguide portionconstituting a lasing cavity used to amplify the optical signal.

(9) A semiconductor optical amplifier according to any of Paragraphs (1)through (8) wherein a variable optical attenuator is integrated at theinput end and/or output end of the optical waveguide which propagatesthe optical signal.

(10) A photo acceptance device with a built-in optical preamplifier,comprising a semiconductor optical amplifier according to any ofParagraphs (1) through (8), provided with a photo acceptance elementintegrated at the output end of the optical waveguide which propagatesthe optical signal.

(11) An optical amplifier module comprising a semiconductor opticalamplifier according to any of Paragraphs (1) through (9) mountedtherein.

(12) An optical reception module in which an photo acceptance devicewith a built-in optical preamplifier according to Paragraph (10) ismounted.

(13) A semiconductor optical amplifier, comprising:

a first optical waveguide which propagates an input optical signal; and

an optical amplification section, which amplifies the optical signal bycausing stimulated emission with:

radiation incident on the first optical waveguide from the directionwhich is included in a plane parallel to the first optical waveguide andintersects the optical propagating direction of the first opticalwaveguide; or

a cavity structure formed in the direction which intersects the opticalpropagating direction of the first optical waveguide.

(14) A semiconductor optical amplifier according to Paragraph (13),wherein any gain medium to amplify light is not provided around theoptical input surface of the first optical waveguide which propagatesthe input optical signal.

(15) A semiconductor optical amplifier according to Paragraph (13),wherein a part or the whole of the gain medium of the first opticalwaveguide which propagates the input optical signal is used also as apart or the whole of the gain medium of the optical amplificationsection which amplifies the optical signal.

(16) A semiconductor optical amplifier according Paragraph (14), whereina part or the whole of the gain medium of the first optical waveguidewhich propagates the input optical signal is used also as a part or thewhole of the gain medium of the optical amplification section whichamplifies the optical signal.

(17) A semiconductor optical amplifier according to Paragraph (15),wherein: a part or the whole of the gain medium of the first opticalwaveguide which propagates the input optical signal is used also as apart of the gain medium of the optical amplification section whichamplifies the optical signal; or the gain medium used in the firstoptical waveguide is different in composition from the gain medium usedin a portion which generates radiation incident on the first opticalwaveguide.

(18) A semiconductor optical amplifier according to Paragraph (16),wherein: a part or the whole of the gain medium of the first opticalwaveguide which propagates the input optical signal is used also as apart of the gain medium of the optical amplification section whichamplifies the optical signal; or the gain medium used in the firstoptical waveguide is different in composition from the gain medium usedin a portion which generates radiation incident on the first opticalwaveguide.

(19) A semiconductor optical amplifier according to Paragraph (13),wherein: a photonic crystal is provided along a side of the firstoptical waveguide which propagates the input optical signal; and with acavity structure formed in the direction which intersects the opticalpropagating direction of the first optical waveguide, the optical signalis amplified by causing stimulated emission.

(20) A semiconductor optical amplifier according to Paragraph (14),wherein: a photonic crystal is provided along a side of the firstoptical waveguide which propagates the input optical signal; and with acavity structure formed in the direction which intersects the opticalpropagating direction of the first optical waveguide, said the opticalsignal is amplified by causing stimulated emission.

(21) A semiconductor optical amplifier according to Paragraph (15),wherein: a photonic crystal is provided along a side of the firstoptical waveguide which propagates the input optical signal; and with acavity structure formed in the direction which intersects the opticalpropagating direction of the first optical waveguide, the optical signalis amplified by causing stimulated emission.

(22) A semiconductor optical amplifier according to Paragraph (16),wherein: a photonic crystal is provided along a side of the firstoptical waveguide which propagates the input optical signal; and with acavity structure formed in the direction which intersects the opticalpropagating direction of the first optical waveguide, the optical signalis amplified by causing stimulated emission.

(23) A semiconductor optical amplifier according to Paragraph (17),wherein: a photonic crystal is provided along a side of the firstoptical waveguide which propagates the input optical signal; and with acavity structure formed in the direction which intersects the opticalpropagating direction of the first optical waveguide, the optical signalis amplified by causing stimulated emission.

(24) A semiconductor optical amplifier according to Paragraph (18),wherein: a photonic crystal is provided along a side of the firstoptical waveguide which propagates the input optical signal; and with acavity structure formed in the direction which intersects the opticalpropagating direction of the first optical waveguide, the optical signalis amplified by causing stimulated emission.

(25) A semiconductor optical amplifier according to Paragraph (13)wherein, without injecting current into the first optical waveguidewhich propagates the input optical signal, the optical signal isamplified by causing stimulated emission by optically pumping carriersin the first optical waveguide with radiation incident on the firstoptical waveguide from the direction which is included in a planeparallel to the first optical waveguide and intersects the opticalpropagating direction of the first optical waveguide.

(26) A semiconductor optical amplifier according to Paragraph (14)wherein, without injecting current into the first optical waveguidewhich propagates the input optical signal, the optical signal isamplified by causing stimulated emission by optically pumping carriersin the first optical waveguide with radiation incident on the firstoptical waveguide from the direction which is included in a planeparallel to the first optical waveguide and intersects the opticalpropagating direction of the first optical waveguide.

(27) A semiconductor optical amplifier according to Paragraph (15)wherein, without injecting current into the first optical waveguidewhich propagates the input optical signal, the optical signal isamplified by causing stimulated emission by optically pumping carriersin the first optical waveguide with radiation incident on the firstoptical waveguide from the direction which is included in a planeparallel to the first optical waveguide and intersects the opticalpropagating direction of the first optical waveguide.

(28) A semiconductor optical amplifier according to Paragraph (16)wherein, without injecting current into the first optical waveguidewhich propagates the input optical signal, the optical signal isamplified by causing stimulated emission by optically pumping carriersin the first optical waveguide with radiation incident on the firstoptical waveguide from the direction which is included in a planeparallel to the first optical waveguide and intersects the opticalpropagating direction of the first optical waveguide.

(29) A semiconductor optical amplifier according to Paragraph (17)wherein, without injecting current into the first optical waveguidewhich propagates the input optical signal, the optical signal isamplified by causing stimulated emission by optically pumping carriersin the first optical waveguide with radiation incident on the firstoptical waveguide from the direction which is included in a planeparallel to the first optical waveguide and intersects the opticalpropagating direction of the first optical waveguide.

(30) A semiconductor optical amplifier according to Paragraph (18)wherein, without injecting current into the first optical waveguidewhich propagates the input optical signal, the optical signal isamplified by causing stimulated emission by optically pumping carriersin the first optical waveguide with radiation incident on the firstoptical waveguide from the direction which is included in a planeparallel to the first optical waveguide and intersects the opticalpropagating direction of the first optical waveguide.

(31) A semiconductor optical amplifier according to Paragraph (19)wherein, without injecting current into the first optical waveguidewhich propagates the input optical signal, the optical signal isamplified by causing stimulated emission by optically pumping carriersin the first optical waveguide with radiation incident on the firstoptical waveguide from the direction which is included in a planeparallel to the first optical waveguide and intersects the opticalpropagating direction of the first optical waveguide.

(32) A semiconductor optical amplifier according to Paragraph (20)wherein, without injecting current into the first optical waveguidewhich propagates the input optical signal, the optical signal isamplified by causing stimulated emission by optically pumping carriersin the first optical waveguide with radiation incident on the firstoptical waveguide from the direction which is included in a planeparallel to the first optical waveguide and intersects the opticalpropagating direction of the first optical waveguide.

(33) A semiconductor optical amplifier according to Paragraph (21)wherein, without injecting current into the first optical waveguidewhich propagates the input optical signal, the optical signal isamplified by causing stimulated emission by optically pumping carriersin the first optical waveguide with radiation incident on the firstoptical waveguide from the direction which is included in a planeparallel to the first optical waveguide and intersects the opticalpropagating direction of the first optical waveguide.

(34) A semiconductor optical amplifier according to Paragraph (22)wherein, without injecting current into the first optical waveguidewhich propagates the input optical signal, the optical signal isamplified by causing stimulated emission by optically pumping carriersin the first optical waveguide with radiation incident on the firstoptical waveguide from the direction which is included in a planeparallel to the first optical waveguide and intersects the opticalpropagating direction of the first optical waveguide.

(35) A semiconductor optical amplifier according to Paragraph (23)wherein, without injecting current into the first optical waveguidewhich propagates the input optical signal, the optical signal isamplified by causing stimulated emission by optically pumping carriersin the first optical waveguide with radiation incident on the firstoptical waveguide from the direction which is included in a planeparallel to the first optical waveguide and intersects the opticalpropagating direction of the first optical waveguide.

(36) A semiconductor optical amplifier according to Paragraph (24)wherein, without injecting current into the first optical waveguidewhich propagates the input optical signal, the optical signal isamplified by causing stimulated emission by optically pumping carriersin the first optical waveguide with radiation incident on the firstoptical waveguide from the direction which is included in a planeparallel to the first optical waveguide and intersects the opticalpropagating direction of the first optical waveguide.

As described in detail so far, the present invention can substantiallyprevent the gain from changing depending on the intensity of the inputoptical signal. Spectrum broadening due to spontaneous emission,accompanied by rising noise level, can also be suppressed according tothe present invention.

In addition, integration of components according to the presentinvention can provide a high function and inexpensive optical amplifier.

Reference numerals are explained as follows:

101 . . . optical waveguide for optical signal, 102 . . . opticalwaveguide region for lasing, 103 . . . electrode pad, 104 . . .antireflection coating film on facet of optical waveguide for opticalsignal, 105 . . . coating film on lasing cavity, 106 . . . variableoptical attenuator, 107 . . . semiconductor optical amplifier, 108 . . .PIN photodiode, 109 . . . preamplifier, 111 . . . InP substrate, 112 . .. InP buffer layer, 113, 115 . . . InGaAsP cladding layer, 114 . . .InGaAsP waveguide layer, 116 . . . InP cap layer, 117 . . . insulationfilm, 118 . . . p-electrode (Ti/Pt/Au), 119 . . . n-electrode(Ni/AuGe/Au), 122, 128 . . . InGaAsP cladding layer, 123, 125 . . .InGaAsP SCH layer, 124 . . . InGaAsP MQW active layer, 126 . . . InPspacer layer, 127 . . . InGaAsP grating layer, 128 . . . InP claddinglayer, 129 . . . InGaAs contact layer, 130 . . . semi-insulation InPburied layer, 131 . . . 45-degree mirror part, 132, 133 . . . highreflectance dielectric film, 134 . . . n-type semiconductormulti-layered film, 153, 155 . . . InGaAsP SCH layer, 154 . . . InGaAsPMQW active layer, 156 . . . InP cladding layer, 157 . . . InGaAs contactlayer, 163, 165 . . . InGaAsP cladding layer, 164 . . . InGaAs opticalabsorption layer, 166 . . . InGaAs cap layer, 167 . . . InGaAs contactlayer, 210 . . . lens, 211 . . . optical fiber, 212 . . . Peltier device

1. A semiconductor optical amplifier, comprising: a first opticalwaveguide which propagates an input optical signal; and an opticalamplification section for carrying out amplification by causinginduction discharge to occur with a resonator structure formed from adirection that, in a plane parallel to the first optical waveguide andsubstrate, intersects the optical propagation direction of the firstoptical waveguide, the optical amplification section amplifying theinput optical signal by causing stimulated emission with radiationincident on the first optical waveguide from a direction which isincluded in a plane parallel to the first optical waveguide and asubstrate and intersects an optical propagating direction of the firstoptical waveguide, and a cavity structure formed in a direction whichintersects the optical propagating direction of the first opticalwaveguide.
 2. A semiconductor optical amplifier according to claim 1,wherein any gain medium to amplify light is not provided around anoptical input surface of the first optical waveguide excepting coatingfilms, which propagates the input optical signal.
 3. A semiconductoroptical amplifier according to claim 2, wherein a part or the whole of again medium of the first optical waveguide which propagates the inputoptical signal is used also as a part or the whole of a gain medium ofthe optical amplification section which amplifies the optical signal. 4.A semiconductor optical amplifier according to claim 3, wherein: a partor the whole of the gain medium of the first optical waveguide whichpropagates the input optical signal is used also as the gain medium usedin the first optical waveguide is different in composition from a gainmedium used in a portion which generates radiation incident on the firstoptical waveguide.
 5. A semiconductor optical amplifier according toclaim 3, wherein: photonic crystals are provided as mirrors of a cavitystructure along both sides of the first optical waveguide whichpropagates the input optical signal; and the optical signal is amplifiedby allowing a cavity structure to cause stimulated emission, said cavitystructure being formed in a plane parallel to the first opticalwaveguide and substrate and in the direction which intersects theoptical propagating direction of the first optical waveguide.
 6. Asemiconductor optical amplifier according to claim 4, wherein: photoniccrystals are provided as mirrors of a cavity structure along both sidesof the first optical waveguide which propagates the input opticalsignal; and the optical signal is amplified by allowing a cavitystructure to cause stimulated emission, said cavity structure beingformed in a plane parallel to the first optical waveguide and substrateand in the direction which intersects the optical propagating directionof the first optical waveguide.
 7. A semiconductor optical amplifieraccording to claim 2, wherein: photonic crystals are provided as mirrorsof a cavity structure along both sides of the first optical waveguidewhich propagates the input optical signal; and the optical signal isamplified by allowing a cavity structure to cause stimulated emission,said cavity structure being formed in a plane parallel to the firstoptical waveguide and substrate and in the direction which intersectsthe optical propagating direction of the first optical waveguide.
 8. Asemiconductor optical amplifier according to claim 2 wherein at leastsome of a gain medium of the first optical waveguide which propagatesthe input optical signal is different in composition from a gain mediumused in a portion which generates radiation incident on the firstoptical waveguide and provided in a plane parallel to the first opticalwaveguide and substrate.
 9. A semiconductor optical amplifier accordingto claim 1, wherein a part or the whole of a gain medium of the firstoptical waveguide which propagates the input optical signal is used alsoas a part or the whole of a gain medium of the optical amplificationsection which amplifies the optical signal.
 10. A semiconductor opticalamplifier according to claim 9, wherein: a part or the whole of the gainmedium of the first optical waveguide which propagates the input opticalsignal is used also as the gain medium used in the first opticalwaveguide is different in composition from a gain medium used in aportion which generates radiation incident on the first opticalwaveguide.
 11. A semiconductor optical amplifier according to claim 10,wherein: photonic crystals are provided as mirrors of a cavity structurealong both sides of the first optical waveguide which propagates theinput optical signal; and the optical signal is amplified by allowing acavity structure to cause stimulated emission, said cavity structurebeing formed in a plane parallel to the first optical waveguide andsubstrate and in the direction which intersects the optical propagatingdirection of the first optical waveguide.
 12. A semiconductor opticalamplifier according to claim 9, wherein: photonic crystals are providedas mirrors of a cavity structure along both sides of the first opticalwaveguide which propagates the input optical signal; and the opticalsignal is amplified by allowing a cavity structure to cause stimulatedemission, said cavity structure being formed in a plane parallel to thefirst optical waveguide and substrate and in the direction whichintersects the optical propagating direction of the first opticalwaveguide.
 13. A semiconductor optical amplifier according to claim 1,wherein: photonic crystals are provided as mirrors of a cavity structurealong both sides of the first optical waveguide which propagates theinput optical signal; and the optical signal is amplified by allowing acavity structure to cause stimulated emission, said cavity structurebeing formed in a plane parallel to the first optical waveguide andsubstrate and in the direction which intersects the optical propagatingdirection of the first optical waveguide.
 14. A semiconductor opticalamplifier according to claim 1 wherein at least some of a gain medium ofthe first optical waveguide which propagates the input optical signal isdifferent in composition from a gain medium used in a portion whichgenerates radiation incident on the first optical waveguide and providedin a plane parallel to the first optical waveguide and substrate.